Enameled articles



Patented Jan. 31, 1 950 Frank R.. Porter Crown Point, Ind., .assig,norto. GeorgeN. Hibben, Chicago, Ill'.,.as trustee.

N'o Drawl'ng; -Application:AzprileZfl, 1949,; Serial No. 90,0.49

This application is a continuationein-part ofcopending applicationSerial. No. 591,603, filed June 4,1945.

This. invention relates to light-colored. enameled'e steel articles and.more: particularly to a means of eliminatin the: use of the customarydark-colored ground coat in enameling, articles. of this character.

In. order to obtain: on. a; steel oriron article. a: smooth uniformlyadhering, coat of. light-coleorect porcelain or vitreous enamel, it has.been the. practice in. the enameling trade: for many years. to. apply orfusezto the steel. article. a dark-colored ground coat containing; anoxide such. as cobalt oxide, nickel oxide or manganese. oxide. Over thedarkecolored ground coat one or more finishing or cover coats of thedesired light-colored enamel were, then applied. and; separately fused.In most instances. because. of, the. in'.-.- herent dark. color of theground coat, sit has been considered necessary. to. employ: severalcover coats of light-colored, enamelih order to' achieve a satisfactorylight-coloredfinishg By, the. term light-colored. I mean white. orpastel shades.

The. conventionalv ground coat is dark or blue: black. in color and. isusually fused to thesteel'. base at temperatures of the'order. of fromabout. 1580? F1. to about I6Q0 F. The dark color of' the ground. coat isdue to thepresence of the cobaItfnickel. or manganese oxides whichpossess the'property of' promoting adherence of the cover coats to. thesteel base. The-cover coats are separately fused over the ground coat attemperatures which may range from about 1 1500" F; toabout 1600F.,'dependent upon the. composition and properties of the enamel. Thecover coats may, of course, be of any color. but most of the two orthree coat articles, for example, parts of stoves orrefrigeratorsareflnished in white or pastel shades.

, The purpose of the dark-colored ground coat istwofold: (l) to securesatisfactory adherenceof the cover coats, and (2) to eliminate orminimize in the finished article such objectionable surface defects asblistering; pitting, blackspecking, and fish-scaling.

According to widely accepted theory, the ad. herence of vitreous enamelto steel is accomplished through an iron oxide layer or film formed onthe steel surface and under the enamel coating. Apparently the ironoxide dissolves in both the steel and enamel at enamel fusing tem-.peratures. This iron oxide may begin to form factory unitorm. adherence.

assoon. as. the, enamel is: applied to. he; S eel surface. because of.the moisture, in the. enamel.

However, the quantity of iron, oxide, so formed;

orrplain. carbon, steellis. not suificient for sail longbeenknownt'opromote porcelain .1.. vitreous. enamel adherence and it isbelieved; that, this.

property is, due. to its, relative, unstability; i. e'.,

it releases its oxy en comparatively easi1y,, par.-

, ticularly at. enameling temperatures and when,

in,...the..presence of. some. element. or compound which. will. unite.with. oxygento form. a more stable oxide. which in this. case; is,irorr. When;

the. cobalt has given. up. its. oxygen. to. the. steel.

at enamelfusing.temperatures, it then combines.

with. oxygen from. the atmosphereand. is; ready again to furnish oxygento the steel; Thecobalt.

therefore. serves as. an. oxygen, carrier. or, in. e1.- fect,.,asacatalyst. Nickel and, manganese oxides. arev known. tov behave in 'a.similar manner..., In, accordance with. the foregoing explanation itwill. be. evident. that the ground, coat. constitutes the medium wherebysati'siaetory bond obtention.

is. realized, between. the, steelhase. andlthe finish or cover coatsoffenamel..

It has. also been. established. that one; ofjthe. principal. causes of.surface defects; in. enameled;

articles. particularly blistering. pitting andv blacksspecking, is the.formation and. evolution of. gaseous carbon. products which are.released; from the steel base during. firin oi the enamel; coats, These.gaseous carbon. products. apparently result from the reaction of the.carbon. in the. steelwithmetallic oxides in the enamel or withhydjrogenwhich is. sometimes formed at enameli'ng. temperatures by thejreduction'Qfwater,

contained. in the enamel The surface defect.

known as fish-scaling is considered to be the result of hydrogenescaping from the enameled metal upon cooling from enameli'ng tempera,-

tures to room temperature; The hydrogen whichmay be formed duringenameling dissolves to a substantial extent in thesteeli at elevated"tem-. peratures; and as the enameled metal. coolsga. supersaturatedcondition exists. with the result; that free hydrogen is released fromsolution, in the steel. The'hydrogen'thus released-tends; to createexcessive pressures under the enamel coating which force the enamel offthe steellin' fish sca'le shaped pieces. "The 'applicationiof a groundcoat. to the'base steel tends to 'trap'the various gaseousproducts whichmay form and thereby minimizes the magnitude and frequency. of surfacedefects of the type described}.

Cobalt. oxide has;

3 However, the use of the ground coat does not completely eliminatesurface defects of the above type and the enameling trade has long beenfaced with the annoying problem of rejecting and reprocessing asubstantial proportion of the found that the use of a ground coatintroduces another type of surface defect known as reboiling. In otherwords, during the firing of the outer cover coats, the underlying andpreviously applied ground coat has a tendency to soften and boil therebydisturbing the adherence and smooth appearance of the cover coats. Inorder to. overcome the reboiling problem, it has been necessary tochoose enamel compositions for the cover coats which can be fired attemperatures appreciably lower than the softening temperature of theground coat.

For the past twenty years or more extensive efforts have been made andconsiderable experimentation carried out by ceramic engineers and othersskilled in the enameling arts with the object in view of obtaining alight-colored enameled steel. article which is free of undesirablesurface defects without the necessity of using the conventionaldark-colored ground coat. Moreover, it has been the hope of theseinvestigators that the elimination of the dark-colored ground coat wouldthereby permit the production of enameled steel articles with only a,single light-colored coat of enamel thereby greatly reducing productioncosts both from the point of view of materials used and the eliminationof one or more process steps. It has been found that articles havingonly a single coat of enamel are highly resistant to chipping andcrazing of' the enamel surface as compared with multi-coate d articleshaving two or three or even a greater number of cover coats. Certain ofthe prior art workers have attempted to solve the problem by devisingspecial compositions for the enamel frit or by employing particulartechniques in the preparation of the enamel slip. Others in the priorart have resorted to nickel fiash or nickel dip techniques for thepurpose of improving bond obtention. However, none of these proposalshas provided a satisfactory solution to the problem of surface defectsin enameled articles, and the enameling trade has. consistently employedthe conventional dark-colored ground coat even whenspecial enamelcompositions or nickeling techniques have also been used. 7 g

Accordingly, the principal object of my invention is to providelight-colored enameled steel articles which can be prepared without theuse of the'conventional dark-colored'ground coat and with only a singlelight-colored enamel coat, which articles are substantially free ofobjectionable surface defects.

, Broadly, I accomplish this object by utilizing a killed steel basehaving alloyed therewith a sufficient quantity of zirconium, columbium,or vanadium to chemically combine with and stabilizethe carbon presentin the steel. These metals posses's'the common chemical character'- 4istic of forming relatively stable carbides. By thus tyingup orstabilizing the carbon content of the steel, the formation of gaseouscarbon products is inhibited with the result that surface defects of thetype hereinbefore described are eliminated.

It is important in the practice of my invention that a killed orsubstantially completely deoxidized steel be employed. Deoxidation maybe accomplished by the use of well known deoxidizing agents such asaluminum, titanium, and the like. If the base steel contains oxygen toany appreciable extent, the zirconium, columbium, or vanadium added asan alloying and carbon stabilizing constituent will tend to reactpreferentially with the oxygen content of the steel and thereby fail toreact completely with the carbon. The steels employed in my inventionshould have carbon contents as low as consistent with good steelmakingpractice since it will be obvious that the lower the carbon content theless alloying metal will be required. In general, the carbon content ofthe steel should not be in excess of about 0.15%. In addition to carbonand the alloying metal, the steels may contain the usual amounts of theother common ingredients such as manganese, phosphorus, sulfur, andsilicon.

The killed steel may be rolled into strips, sheets, or plates and thedesired articles formed therefrom by methods well known in the art. Thesteel articles are then cleaned by any desired method which will insurea proper surface for adherence of the enamel coat, and a light-coloredenamel is then applied directly to the steel base and fused thereto byfiring at an appropriate temperature. As hereinafter described ingreater detail, the present invention is directed particularly to theuse of a light-colored enamel containing an adherence-promoting oxideselected from the group consisting of the oxides of antimony,molybdenum, and arsenic. Preferably, the adherence-promoting oxide isincorporated as part of the enamel frit.

If desired, the steel article may be subjected to :a nickeling process,by methods well known in the art, whereby to deposit upon the steel anickel film which has the effect of assisting bond obtention and also oflowering the requisite firing temperature for light-colored enamels ofthe abovementioned type. Such a nickel deposit may comprise from about.02 gram to about .08 gram of nickel per square foot of surface althoughgreater amounts may also be used, if desired. The use of a nickeltreatment prior to the application of the enamel coat to the steelarticle permits the firing temperature of the enamel coat to be reducedby as much as 40 F. to 50 F. below the usual firing temperature requiredfor enamels of this type. Thus, in the case where it is desired toemploy the nickeling pretreatment, the steel base comprises the nickeledsteel article, and the lightcolored enamel coat is applied directly tothe base without the conventional dark-colored ground coat.

As mentioned above, the alloying metal, i. e. zirconium, columbium, orvanadium, is employed in sufficient quantity to combine chemically withthe carbon content of the particular steel being used. On the basis ofexperimental investigation, it has been determined that in the case ofzirconium and colurnbium the weight ratio of alloying metal to carbonshould be within the range of from about 7:1 to about 15:1. In the caseof vanadium, the weight ratio of vanadium to 'cara sess? :bon should bewithinmthe range oftfromwabout 8:1 to about 1 18:1. Thet-upperzvlimits:onithe amounts of alloying metal are determined primarily by economicconsiderations, but for all .practical purposes the above-indicatedmaximum quantities will suffice. Furthermorepbythepse of the alloyingmetal in the above amounts, sufii- Jcient Zirconium, columbium, orvanadium ispro- -vided to compensate for the usual processlossesinzexcess of the stoichiometric quantitiesrequired, rand also suchamounts will be sufficient t0 ..com- -.bine with any free hydrogen whichmaybe formed incidentally duringfiring of the enamel coatings aspreviously described.

I, have found by extended experimentsthat @there is an excellentcorrelation between the amount of alloying metal needed to eliminatesurface defects during subsequent enameling and the:amount necessary toeliminate the yield point ofthe steel as indicated by the conventional,

:stress-strain curves. Accordingly, the yield point :ItBSt affords ahighly convenient physical tech- :nique for determining the appropriateamount of alloying metal for stabilizing the carbon content of any givensteel. Thus, in order to practice my invention, the amount of alloyingmetal to be added to a given quantity of steel may be ascertained byeither of two methods:

(1) The carbon content or the steel maybe determined quantitatively bychemical or metallurgical methods and the quantity of alloying metalthen calculated by selecting a Weight ratio of alloying metal to carbonWithin the ranges disclosed above; or

(2) A series of sample steels may be prepared containing difierent knownquantities of alloying metal and the samples then tested for yield pointby conventional test methods. The samples in which the yield point hasbeen substantially eliminated contain suflicient alloying metal ,to

- stabilize the carbon content and consequently any steel ofcorresponding composition will enamel satisfactorily without theconventional ground coat.

It will thus be seen that in practicing my invention the quantity ofalloying metal to be added to a batch of steel in any given instancemay. be;determined in advance by either purelychemical or by essentiallyphysical methods.

Although the advantages of enameling. steels 50 containing zirconium,columbium or vanadium. in the amounts herein disclosed are obtained-witha wide variety of enamel compositions, my invention is particularlydirected to the use of lightcolored enamel compositions containing aspart of ..the' frit an adherence-promoting oxide selected :from thegroup consisting of the oxides of anti- :.mony, molybdenum, and arsenic.Thus, the .in-

: vention contemplates the use of a steel base con- 1 taining any or"the aforementioned alloying metals (zirconium, columbium, vanadium) :incombina- .tion with an enamel coat containinganyrofithe --previouslymentioned adherence-promoting oxides (antimony, molybdenum, and arsenicoxides).

In order to further illustrate the invention, the following specificexamples are presented, but it will be understood that thescop oftheinvention is by no means limited to-the details of these-car amples.

EXAMPLEI A zirconium steel having a weight ratio of ezzirconium tocarbonof 7.7 :1:-' was,-:enamel'ed;- dieu'ectly, i-. e. without-an.interveninggroundsooat,

using aswhitezenamel containing. antimony oxide as theadherence-promoting;oxide.

,flha analysis of the steel was as follows (the ;balance being iron):

Specimens of thissteel were hot-rolledto one- ,-.quarter 3inch-platesand-samples were cut for enameling. Thespecimens were cleanedthoroug-hly, pickled; dried, and then enameled. The

enamel frit had .the following composition:

Weight percent SbzOs 6.45 SiOz 42.22 A1203 1.80 AlF3 10.00 C'aFz 7.15

ZnO 3.15 'JNaO 12,11 ,K'zO 2.35 "B 14.48

Undetermined portion 0.29

The enamel slip was prepared by combining the following mill additions:

,, Frit(having-above'analysis) grams 3,400 -qoiay do 238 Commercialopacifier do 68 Sodium nitrite do 4.25 Water cc 1,400

The composition of the commercial opacifier re- "..ferred to above wasas follows: ZrOz 79.21 SiO-Z 14.20 %','Na'2O 2.23%,2120 25%, Geo 2.0 3%,F'ezOz 1.03%, A1203 .63%, and undetermined portion The enamel wasapplied to the surface of the "steel samples'by'spraying, the thicknessof the "sprayed coating varying from .006 inch to .010 "inch-Thespray'edenamel coating was dried and fired for approximately six minutes atabout 1560" F.

The samples resulting from this treatment were free of blisters,black-specks, pin-holes, and fishscaling, and the enamel had anexcellent bond, 5 i: e. a high" degree of adherence to the steel sur-"face.

EXAMPLE II Following substantially the same procedure as described aboveinconnection with Example I, a Fzirconium-containing steel having azirconium to carbon ratio of approximately 9.6:1 was enameled with asomewhat similar white enamel con- ;tainingantimony oxide as: theadherence-promotwingoxide.

The analysis of the steel was as follows (rewmainder .being iron plussulfur, phosphorus, and

other impuritiesrin the *usual amounts) Weight per cent Carbon .05Manganese .21 Silic on. 1.62 ;-;Aluminum .026

nmzirconium p.48

enamel frit was sub- An enamel slip was prepared using the above sameproportions as disclosed in connection with Example I. The enamelcoating was applied directly to the steel without the use of anintervening ground coat. Excellent adherence was obtained with noobjectionable surface defects.

EXAMPLE III In place of the zirconium-containing steels such frit andother mill additions in substantially the all?) as those described inExamples I and II, my inven- I tion also contemplates the use of steelscontaining columbium in similar amounts. Typical steel compositionswhich I' have found will enamel satisfactorily without the use of anintervening ground coat are as follows:

Sample No l 2 3 Carbon (weight percent) .05 .07 .09 Manganese .28 .32 34Phosphorus .011 ll .ili'i Sulfur I 026 028 017' Silicon .04 .06 .07Oolumbium. .37 .01 1.20 Aluminum 011 084 .015 Cb/C (weight ratio) 7. 48. 7 13.4

Any of the light-colored enamels containing antimony,

EXAMPLE IV In addition to the zirconiumand columbiumcontaining steels,the invention also embraces the use of vanadium-containing steels ofwhich the following steel analyses are typical of those which. have beenfound to enamel satisfactorily with only a single light-colored enamelcoat applied directly to the steel base:

If; (i

typical molybdenum oxide enamel frit analysis and a slip compositionusing this frit:

Frit

Weight Percent Sodium oxide, NazO 12.04

Potassium oxide, K 3.58 Barium oxide, BaO 23.40 Alumina, A1203 3.83Boric oxide, B203 8.87 Silica, S102 30.33 Antimony oxide, Sb203 2.90Calcium fluoride, CaFz 9.95 Molybdenum oxide, M003 5.10

Mill additions Paris Frit 100.00 Clay 6.00 Sodium nitrite 0.25O-pacifier 4.00 Water 45.00 Other typical molybdenum oxide enamels whichmay be employed are as follows, the ingredients being given on the basisof the smelter batch percentage used in producing the irit:

s it B t 11, w 1 m Ingredient me f 0 g Borax (hydrous) 10. 0 20. 0 20. s20. 5 eldspnr 10. 0 11. 5 22.2 10. 3 Quartz 13.1 17.4 10. 0 12.6 oda Ash.3 8.7 Sodium nitrate .3 3. 5 3. 0 Fiuorspar .8 6.4 4.1 Cryolite 4. 44.9 Galcspar 3.5 Barium carbonate... l7. 2 Zincoxidemnuu- 1.4 Molybdicoxide. 4.1 Antimony trioxid 2. 0 Titanium dioxide 1.4 100.0 100.0 100.0100.0

EXAMPLE VI In place of the molybdenum oxide and antimony oxide enamelshereinbefore disclosed, arsenic oxide-containing enamels may also beemployed in practicing the invention. A typical arsenic oxide enamelfrit and a typical slip composition are as follows: a

i k Frit Sample-No. i 1 5 i 0 7 8 I i 1 Weight PercentCarbon(weigiitpercent).\ .05 .10 .04 .05 4 sodium oxlde Nazo 17373%ihang:i1le se I Potassium oxide, K20 4.80 sufiiijiffi1131:111 183i .032i031 1058 022% Alumina, A1203 Silicon... .0 5 .00 255 W Boric oxide,B203 23.60 :m. .-i- "I \l il l' l l i i l uln .0021 .001 .014 .02 .015slhcai S102 38-22 e (weight ratio) 8.0 1 11.7 I 12.0 15.0 17.0 Arsenicoxide, AS203 2,42 1 i Calcium fluoride, CaFz 8.03

The foregoing steels may be enameled successfully without the use of aground coat by loo-00 v means of any of the light-colored enamels hereinMm additions disclosed containing antimony, molybdenum or arsenic oxidesas part of the frit. Part3 Frit 100.00 EXAMPLE v Clay 7.00 In Examples Iand II, typical antimony-con- 352mg taining enamel compositions weregiven. In Water place of such antimony enamels, molybdenum o trioxideenamels may also be employed with other typical arsenic-containingenamel frits equally satisfactory results. The following is a which maybe used are as follows, the ingredients being reported onztheibasisvofthe smelter batch p rcentages employed in forming theirits:

Ingredient:

Borax Quartz 'Oryolita: 6.7 Arsenic oxide l 4. 6 4. 9

'Iin oxide 9. White lead It will be seen that my invention provides ameans of eliminating the conventional dark-colored ground coat fromenameled steel articles. This result is accomplished by the combinationof a particular steel base with a preferred class of light-coloredenamels. The presence of zirconium, columbium, or vanadium in thedisclosed amounts results in substantially complete chemical combinationof the carbon present in the steel to form stable carbides, and inaddition, any free hydrogen formed during enameling may react with thealloying metal to form the corresponding hydride. Thus, the formationand release of gaseous products during the enameling step is avoided andthe cause of such surface defects as blistering, pitting, black-speckingand fish-scaling is eliminated.

The elimination of the conventional dark-colored ground coat offers anumber of important advantages. Obviously, the process becomes moreeconomical as a result of the saving in material and also as a result ofthe elimination of a process step. Moreover, the conventional groundcoat is fired at a relatively high temperature whereas the cover coatsare usually fired at somewhat lower temperatures. By eliminating theapplication of the ground coat altogether, I avoid subjecting theenameled articles to excessively high temperatures thereby minimizingsagging and warping of the articles during the firing cycle. Inaddition. with the dark-colored ground coat omitted, I am able to employa relatively thin enamel coat which is much more resistant to crazingand chipping than the thick multi-coat wear of the prior art.

I claim:

1. A light-colored enameled steel article comprisin 1) a steel basecomprising a killed steel containing alloyed therewith a metal selectedfrom the group consisting of zirconium. columbium. and vanadium. thecarbon content of said steel being not in excess of about 0.15% and thewei ht ratio of alloying metal to carbon present in the steel being fromabout 8:1 to about 18:1 in the case of vanadium and from about 7:1 toabout :1 in the case of zirconium and columbium. whereby to providesuflicient alloying metal to eliminate the yield point of the steel:and. (2) a coat of light-colored vitreous enamel applied directly tosaid base and containing as part of the frit a light-colored adherencepromoting oxide selected from the group consisting of the oxides ofantimony, molybdenum, and arsenic.

2. A light-colored enameled steel article comprising: (1) a steel basecomprising 2. killed steel containing alloyed therewith a metal selectedrom the group consisting of zirconium, columbium, and vanadium, thecarbon content of said steel being not in excess of about 0.15% and theweight ratio of alloying metal to carbon present in the 1 steel-being.from about 8:1 -to about 18: 1 -:in the case of vanadium and from about7:1 to

about 15:1 in the case ofzirconium and columbium; and (2) a coat oflight-colored vitreous enamel applied directly to said base and con*tainingas part of the frit a light-colored adherence promotingoxideselected from the group consisting of the .oxides .of antimony,molybdenum, and arsenic.

-3. A light-:colored enameled steel article comprising: 1) a steel basecomprising a killed steel containing vanadium alloyed therewith, thecarbon content of said steel being not in excess of about 0.15% and theweight ratio of vanadium to carbon present in the steel being from about8:1 to about 18:1; and (2) a coat of light-colored vitreous enamelapplied directly to said base and containing as part of the frit alight-colored adherence promoting oxide selected from the groupconsisting of the oxides of antimony, molybdenum, and arsenic.

4. A light-colored enameled steel article comprising: 1) a steel basecomprising a killed steel containing zirconium alloyed therewith, thecarbon content of said steel being not in excess of about 0.15% and theweight ratio of zirconium to carbon present in the steel being fromabout 7:1 to about 15:1; and (2) a coat of light-colored vitreous enamelapplied directly to said base and containing as part of the frit alightcolored adherence promoting oxide selected from the groupconsisting of the oxides of antimony, molybdenum, and arsenic.

5. A light-colored enameled steel article comprising: 1) a steel basecomprising a killed steel containing columbium alloyed therewith, thecarbon content of said steel being not in excess of about 0.15% and theweight ratio of columbium to carbon present in the steel being fromabout 7:1 to about 15:1; and (2) a coat of lightcolored vitreous enamelapplied directly to said base and containing as part of the frit alightcolored adherence promoting oxide selected from the groupconsisting of the oxides of antimony, molybdenum, and arsenic.

6. A light-colored enameled steel article comprising: (1 )a steel basecomprising a killed steel containing alloyed therewith a metal selectedfrom the group consisting of zirconium, columbium, and vanadium, thecarbon content of said steel being not in excess of about 0.15% and theweight ratio of alloying metal to carbon present in the steel being fromabout 8:1 to about 18:1 in the case of vanadium and from about 7:1 toabout 15:1 in the case of zirconium and columbium; and (2) a coat oflight-colored vitreous enamel applied directly to said base andcontaining antimony oxide as part of the frit.

7. A light-colored enameled steel article comprising: (l) a steel basecomprising a killed steel containing alloyed therewith a metal se lectedfrom the group consisting of zirconium, columbium, and vanadium, thecarbon content of said steel being not in excess of about 0.15% and theweight ratio of alloying metal to carbon present in the steel being fromabout 8:1 to about 18:1 in the case of vanadium and from about 7:1 toabout 15:1 in the case of zirconium and columbium; and (2) a coat oflight-colored vitreous enamel applied directly to said base andcontaining molybdenum oxide as part of the frit.

8. Alight-colored enameled steel article comprising: (l) a steel basecomprising a killed steel containing alloyed therewith a metal selected5 from the group consisting of zirconium, columin the case of vanadiumand from about 7:1 to.

about 15:1 in the case of zirconium and columbium; and (2) a coat oflight-colored vitreous enamel applied directly to said base andcontaining arsenic oxide as part of the frit.

FRANK R. PORTER.

REFERENCES CITED The following references are of record in the file ofthis patent:

5 UNITED STATES PATENTS Number Name Date 1,360,317 Rieser Nov. 30, 19202,396,856 King Mar. 19, 1946 2,414,633 Bryant Jan. 21, 1947

1. A LIGHT-COLORED ENAMELED STEEL ARTICLE COMPRISING: (1) A STEEL BASECOMPRISING A KILLED STEEL CONTAINING ALLOYED THEREWITH A METAL SELECTEDFROM THE GROUP CONSISTING OF ZIRCONIUM, COLUMBIUM, AND VANDIUM, THECARBON CONTENT OF SAID STEEL BEING NOT IN EXCESS OF ABOUT 0.15% AND THEWEIGHT RATIO OF ALLOYING METAL TO CARBON PRESENT IN THE STEEL BEING FROMABOUT 8:1 TO ABOUT 18:1 IN THE CASE OF VANADIUM AND FROM ABOUT 7:1 TOABOUT 15:1 IN THE CASE OF ZIRCONIUM AND COLUMBIUM, WHEREBY TO PROVIDESUFFICIENT ALLOYING METAL TO ELIMINATE THE YIELD POINT OF THE STEEL; AND(2) A COAT OF LIGHT-COLORED VITREOUS ENAMEL APPLIED DIRECTLY TO SAIDBASE AND CONTAINING AS PART OF THE FRIT A LIGHT-COLORED ADHERENCEPROMOTING OXIDE SELECTED FROM THE GROUP CONSISTING OF THE OXIDES OFANTIMONY, MOLYBDENUM, AND ARSENIC.